Compensating device used with different sized cartridges



United States Patent COMPENSATING DEVICE USED WITH DIFFERENT SIZED CARTRIDGES Roger Marsh, Hudson, Ohio, assignor to Weapons, Inc... Hudson, Ohio, a corporation of Ohio Application March 16, 1955, Serial No. 494,754

1 Claim. (Cl. 89-193) The present invention relates to a compensating device for self-actuating firearms, such as semi-automatic rifles, subrnachine guns, machine guns, the so-called automatic pistols, and/or other automatic or semiautomatic firearms, and more particularly to an auxiliary motor means for a firearm which compensates for differences in the actuating power or actuating energy of the rounds of ammunition in accordance with differences in the size of the cartridge case. I

In my copending applications Serial No. 390,915, filed November 9, 1953, now abandoned and Serial No. 461,999, filed October 13, 1954, entitled Compensating Device for Self-Actuating Firearms, now Patent No. 2,865,256, a firearm is disclosed having a compensating piston or piston-valve with valve portions which indicate the position of the movable parts of the main actuating mechanism of the gun. Such valve portions may control the auxiliary compensating motor in accordance with the initial position of such parts before firing and/or the position of such parts when the projectile has moved a predetermined distance to compensate for differences in the actuating power of the rounds of ammunition.

The present invention relates to a new and improved compensating device for self-actuating firearms of the type disclosed in the aforesaid copending applications. However, unlike the compensating devices disclosed in those applications, the compensating device of this invention is not concerned with the positions of the parts of the main actuating mechanism of the gun before or after firing but instead is concerned with the position of the forward portion or mouth of each cartridge case during the travel of a projectile through the barrel.

According to the present invention an auxiliary motor is provided which compensates for variations in the explosive power of each round of ammunition accompanied with changes in the size of the cartridge case even though. the bases of cartridge cases of different lengths each have approximately the same position relative to the barrel at the instant of firing of the cartridge, as for example is the case in firearms adapted to fire rimmed cartridges wherein the cartridge base has a diameter larger than the internal diameter of the firing chamber and the rim or margin of the cartridge base engages the mouth of 'the'chamber to limit the extent of forward movement of the cartridge case into the firing chamber, a system nor-- mally described in the art as chambering on the rim or as seating on the rim of the cartridge case.

' The auxiliary compensating motor preferably com-- prises a cylinder and a booster piston which receives motive fluid from the interior of the barrel through a gas passage near the forward end of the firing chamber. Such.

gas passage is located near the front of a cartridge case which is in firing position so as to be closed by the mouth of a long cartridge case during firing and during passage of. the bullet or projectile through the barrel until the pressure in the barrel has been reduced substantially and "so as to be open before the pressure has been so reduced :when a cartridge with a short case is fired. Since the "ice relative movement between the barrel and the cartridge case is very small before the bullet approaches the end of the barrel and before the pressure in the barrel drops to a relatively low value, the gas port is located immediately adjacent to the position of the mouth of a relatively short cartridge case when fully entered into the cartridge chamber and a short distance rearwardly of the forward end or mouth of a relatively long cartridge case so that high pressure gases may be admitted to the compensating motor when a relatively shorter cartridge case is used but so that high pressure gases will not be admitted to the compensating motor during the period of high pressure in the barrel when a relatively longer cartridge case is used. The auxiliary compensating motor automatically compensates for changes in the explosive power of the cartridges which are accompanied with changes in the lengths of the cartridge cases without requiring manual adjustments by the operator.

An object of the present invention is to provide a com pensating device for a self-actuating firearm which reduces malfunctions due to'varia-tions in the explosive power of difierent cartridges.

Another object of the invention is to provide a simple compensating device for a self-actuating firearm which is inexpensive and easy to manufacture.

Other objects, uses and advantages of the invention will become apparent to those skilled in the art from the following description and from the drawings, in which:

Figure 1 is a schematic side elevational view with parts broken away of a simple self-actuating firearm of the blowback type incorporating the compensating device of the present invention;

Figure 2 is a fragmentary longitudinal vertical sectional view of the gun shown in Fig. 1 on a larger scale with a portion of the bolt broken away and shown in section, the gun being shown with a cartridge therein having a short cartridge case;

Figure 3 is a fragmentary longitudinal vertical sectional view similar to Fig. 2 but showing the gun with a more powerful cartridge therein having a long cartridge case; and

Figure 4 is a fragmentary longitudinal vertical sectional view similar to Fig. 2 but showing a modified form of compensating device, the gun being shown with a cartridge therein having a short cartridge case.

Referring more particularly to the drawings in which like parts are identified with the same numerals throughout the several views, Figs. 1 to 3 illustrate in schematic form a simple blowback firearm which incorporates a compensating device that is designed to modify the operation of the main actuating mechanism of the selfactuating firearm in accordance with the lengthof the cartridge case of each round of ammunition.

The blowback firearm includes a receiver 1, a forwardly extending barrel 2 rigidly connected to the receiver, an externally cylindrical bolt 3 mountedfor longitudinal reciprocation in the receiver parallel to the axis of the barrel, a driving spring or bolt return spring 4 for yieldably resisting rearward movement of the .bolt

and for moving the bolt forwardly to feed a new cardevice of this invention, the self-actuating firearm may which said actuating mechanism was designed.

' cartridge.

be of aconventional type which can fire cartridges automatically or semiautomatically in the intended manner without substantial malfunctions when the cartridges used have the proper explosive power.

According to the present invention a self-actuating firearm is provided with a compensating device in the form of an auxiliary motor which modifies the action of the main actuating mechanism of the gun when cart ridges are fired having abnormal explosive'power and which is ineffective when normal cartridges are fired for Such motor is rendered effective or ineffective in accordance with the size or shape of the cartridges and preferably comprises a fluid motor having a compensating piston which receives motive fluid from the barrel through a gas port which port is open or closed depending on the length of the cartridge case.

It will be apparent that the compensating device of the present invention may be applied to various selfactuating firearms including blowforward firearms, gasoperated firearms and other automatic and/ or semi-automatic firearms mentioned in the aforesaid copending applications. However, for convenience in illustrating, the compensating device is'shown as applied to a simple selfactuating firearm of the blowback type.

It will be understood that the term actuating energy or actuating power, as used in this application, has the same meaning as in the aforesaid copendingapplications and has been employed to describe the portion of the power or energy from each round of ammunition used to perform various functions in the operating cycle such as loading, obturating, extracting, cocking, firing, and/or the like. Thus, in a blowback gunof the type illustrated herein, the actuating power is the pressure of the powder gases thrusting backward against the bolt, and in a blowforward gun is the pressure of the powder gases acting through the projectile to drag the barrel forward.

As will be apparent from the drawings, the barrel is provided with a conventional bore 6 and a conventional cylindrical firing chamber 7 at the breech end of the barrel coaxial with said bore and optionally of a larger diameter so that an annular shoulder 8 is provided in the barrel between the bore and the counterbore, said firing chamber receiving cartridges fed from the magazine at 5. The shoulder 8 preferably has the characteristics of a forcing cone and may be optionally sharp as shown in the drawings. In the event that the firearm is to use ammunition of types normally using so-called heeltype bullets, in which the major diameter of the bullet is approximately equal to the major diameter of the cartridge case body at the mouth, the firing chamber 7 will approximate the bore 6 in dimension, and the shoulder 8 will be minimized or may even be entirely absent. Similarly, the firing chamber 7, shown herein as being cylindrical, may be of any internal profile required to accept the ammunition for which the specific firearm may be designed. The bore 6 extends forwardly from the firing chamber to the front end of the barrel and is preferably provided with conventional rifling which extends throughout the major portion of the length of the barrel to guide the bullet or projectile through the barrel.

As herein shown, the bolt 3 is provided with a circular recess 9 coaxial with the bore 6 and of a size to receive the base of each cartridge fed to the barrel, but it will be understood that, if desired, the breech end of the barrel may be shaped to receive such cartridge base in which case the bolt recess 9 could be omitted.

As shown in the drawings, 10 is a standard .38 Special cartridge and 10a is a standard .357 Magnum cartridge which is substantially more powerful than the former The cartridge 10 comprises a bullet or projectile 11 and a short cartridge case 12 having a thinwalled cylindrical portion 13 with an external diameter substantially equal to the internal diameter of the firing vided adjacent the face 15 of the cartridge base.

chamber7 and a length substantially less than that of 7 The margin of the base 14 provides a rim with a flat annular face 15 which engages the flat rear face 16 of .the barrel around the periphery of the firing chamber 7 to limit forward movement of the cartridge case 12 into the firing chamber. An annular groove 17 is pr}? T e bullet 11 fits tightly in the thin-walled cartridge portion 13 and has an external diameter slightly less than that of said portion 13 so as to fit properly in the bore 6 when the gun is fired. The cartridge case 12 contains a. suit able propellant powder and a suitable primer adjacent the center of the base 14 to facilitate ignition of said propellant. The case 12 has a forward annular edge 18 at its mouth that is a substantial distance to the rear of the annular shoulder 8 when the cartridge 10' is in firing position withthe face 15' in engagement with the barrel face 16 and'that is a'substantial distance in front of the rear face of the bullet 11.

The cartridge 101: comprises a bullet or projectile 110, shown herein as being the same size and shape as the bullet 11, and a long cartridge case 12a having a thin walled cylindrical portion 13a with a forward annular edge 18a. The long cartridge case 12a has a base 14, a barrel-engaging face 15 and an annular groove 17 and is identical with the short cartridge case 12. except that it has a greater length and contains a more powerful propellant. As herein shown, the bullet has substantially the'same position relative to the base 14 in the cartridge 10a as in the cartridge 10 so that the internal volumes of the two cartridges may be substantially the same, but it will beapparent that the gun can fire cartridges wherein the front of the'bullet is farther ahead of the front end of the long cartridge case and the case contains a larger volume of propellant. Regardless of the volume of propellant, the cartridge case may have a length which is substantially proportional to the explosive power of the propellant therein or proportional to the actuating power which normally would be provided by the cartridge in the absence of a compensating device.

The firing chamber 7 has an axial length sufiicient to receive both long and short cartridge cases and, as shown herein, has a length only slightly greater than that of the cylindrical portion 13a of the long cartridge 10a. The forward edge 18a of the long cartridge case is therefore near or substantially in engagement with the annular shoulder 8 of the barrel when the face 15 of the cartridge rim engages the face 16 of the barrel.

The self-actuating firearm with the elements 1 to 16 has an automatic actuating mechanism designed to operate with the long cartridges 10a and can fire such cartridges autoamtically at a cyclic rate or speed determined by the designer according to the requirements of the service of the firearm but, at any required speed, without malfunction, and, optionally, semi-automatically if desired. The firearm can load, fire, extract and eject automatically like conventional blowback firearms so long as the cartridge has the right size and the proper power. However, the conventional firearm has a tendency toward malfunctions when rounds of ammunition are used which do not provide the amount of actuating power for which the firearm was designed.

The barrel shown is of such a size that both .38 Special and .357 Magnum ammunition may be fired therein. The bullet 11 of the .38 Special cartridge is frequently made of lead for police service use but may be found with a partial or full jacket of harder metal such as brass or gilding metal. The bullet 11a of the .357 Magnum cartridge is normally found with a full or partial jacket of harder metal such as brass or gilding metal but may also be found of naked lead or lead alloy. Upon occasion, both cartridges may be found loaded with special projectiles made of such alloys as Kirksitc or other zinc assume alloys, sometimes coated with other materials. Indeed,

the projectiles may be found made of any appropriate .substances or combinations of substances so long as the bullet diameter approximates a standard (in the case of, the .38 Special and .357 Magnum this will be in the vicinity of .357 maximum outside diameter) and so long asbullet energy approximates the standard for the round.

The firearm shown in Figs. 1 to 3 is designed to operate effectively without the help of the compensating device when the .357 Magnum cartridges are employed. In order to permit effective use of .38 Special cartridges without the necessity of manual adjustments or the like, the self-actuating firearm is provided with a compensating device 19 which is rendered ineffective whenever the highpower cartridge 10a is fired and which is rendered effective whenever the less powerful cartridge 10 is fired. Such compensating device is responsive to the explosive powerv of the cartridges and preferably modifies the operation of the main actuating mechanism of the gun in response to the length of the cartridge cases. Such cases may be made with lengths which indicate the actuating power normally obtained therefrom during firing so that the compensating device is controlled in accordance with said actuating power.

The compensating device 19 comprises an auxiliary motor for modifying the operation of the main automatic actuating mechanism of the gun and preferably comprises a fluid motor for receiving motive fluid from the barrel of the gun. As herein shown, the compensating device comprises an auxiliary pneumatic reciprocating motor 20 having relatively movable parts including a gas cylinder housing 21 and a compensating piston 22 and having a gas port 23 for supplying high pressure propellant gases to the compensating piston from the interior of the barrel. One of said relatively movable parts (i.e., the piston as in Figs. 1, 2 and 3, or the cylinder as in Fig. 4) is operably connected to the main actuating mechanism of the self-actuating firearm to impart a substantial force thereto whenever high pressure gases are admitted to the compensating motor from the barrel. As herein shown, the gas cylinder 21 is rigidly connected to the receiver 1 and the barrel 2 and the piston 22 is operably connected to the bolt 3 of the main actuating mechanism of the gun.

The compensating piston may be connected to the bolt for movement in unison therewith, for example substantially as shown in the aforesaid copending applications, or may be integral with the bolt; but, as herein shown, the piston 22 is detached from the bolt and is prevented from moving as far as the bolt by a cylindrical cross pin or stop pin 24. If the piston is integral with the bolt, the cross pin will, of course, be omitted.

As shown herein for purposes of illustration, the housing is integral with the receiver 1 and the barrel 2, but it will be apparent that the compensating motor 20 may take various other forms and that the motor housing may be arranged in a quite different manner. The housing 21 isprovided with a cylindrical bore 25 with an axis parallel to that of the bore 6 and with a length greater than that of the booster piston 22 so as to provide a pressure chamber or booster chamber 26 in the housing 21 forwardly of the piston. The booster piston 22 has an external cylindrical surface substantially throughout its length which is substantially the same diameter as the diameter of the bore 25 so as to prevent any substantial leakage of gases from the booster chamber 26 and is provided with labyrinth grooves 27 to reduce the leakage of high. pressure gases from said chamber. As herein shown, the booster, piston has fiat parallel front and rear faces 28 and 29 perpendicular to the axes of the bores 6 and 25 and parallel to the circumferential grooves 27.

A slot 30 is provided in the bottom of the booster piston 22 rearwardly of the grooves 27 to receive the cylindrical cross pin 24 and to permit limited movement ofthe piston axially in the bore 25. As herein shown, the

cross pin has its axis located in perpendicular planes one of which is perpendicular to the axis of the bore 25 and the other of which is substantially tangent to the cylindrical surface of said bore. The slot 30 therefore has a maximum depth substantially equal to the radius of the cross pin 24 and has cylindrically curved end portions with a radius of curvature substantially equal to the. radius of said cross pin whereby the booster piston 22 engages about one-fourth the circumference of the pin 24 at each end of its stroke.

Although the self-actuating firearm is shown diagrammatically, it will be understood that the compensating device 19 may have relative dimensions substantially as f illustrated in Figs. 1 to 3 since the compensating device. 7

19 and the cartridges 10 and 10a are drawn substantially to scale. Since the piston 22 is shown as being shorter than the .38 Special cartridge case 12, it will be apparent that such piston may be designed with a length of about one inch or substantially less than one inch depending on tht firearm in which it is employed. In a gun of the type shown herein, a compensating piston with a diameter of about one-half inch can provide adequate boost.

As shown herein, the bolt 3 has a flat face 31. perpendicular to the axis of the barred 2 and parallel to the rear faces 16 and 29 of the barrel 2 and the compensating piston 22, respectively, and parallel to the flat bottom 1 surface of the recess 9. The face 31 is spaced from .the.

bottom surface of said recess a distance. corresponding to the thickness of the cartridge base 14 so that it engages the barrel face 16 when the rim surface 15 engages said face and the bolt is at the forward end of its stroke. As

shown in the drawings, the cross pin 24 engages the cylindrical surface of the slot 30 when the booster piston 22 is at the forward limit of its travel so as to position the rear face 29 of the piston flush with the barrel face 16 where it may engage the bolt face 31 at the instant .of firing. Such a construction avoids the unnecessary hammering action which Wouldoccur if the faces 29 and 31.

were spacedapart at the instant of firing.

The compensating device 19 includes means for supthroughthe housing 21, the resulting opening iii- ,31. housing being tapped and permanently closed by a...

threaded plug 32. The port 23 is located forwardly of the piston 22 at all times and connects the booster chamber 26 to the firing chamber near the forward end of the firing chamber 7 and rearwardly of the annular shoulder 8. The port is spaced .a substantial distance from said shoulder so that it may be closed by a relatively long cartridge case during the travel of a bullet or projectile through the barrel.

Since the compensating device 19 is not needed during firing of the cartridges 10a for which the main actuating mechanism of the gun was designed, the port 23 is 10- cated to the rear of the edge 18a when the cartridge case 12a is fully chambered andthe surfaces 15 and 16 are? in engagement so as to be closed by the mouth of the f cartridge case. The port should be a substantial distance rearwardly ofthe edge 18a so that it will not be opened until the high pressure of the powder gases .has been reduced to a point where the gases cannot impart substantial force to the bolt through the booster motor 20. Such distance dependsvon the length of the barrel, on i the type and size of gun, on the design of the cartridge and on the interaction of these and other design factors, and hence it must be determined anew for each application of the basic design.

Since the booster effect of the compensating device'19 1' is needed during the firing of cartridges having less ex plosive power than the cartridges 10a for which the main 7 actuating mechanismof' the gun' wasdesigned, the weaker cartridges are provided with shorter cartridge cases or cases of special shape which open the gas'port 23 during the travel of the bull'et'or projectile through the barrel before the high pressure in th"barrel drops materially and preferably before thepressure drops below about eighty percent of its peak value. forward edge 18 of the short cartridge case 12'is just behind the port 23 and said port isopen'when'th'e faces 15 and 16 are in engagement and said cartridge 'case is in firing position. However, the port may be moved to a position a short distance behind its'position shown in the drawings so long as the port is opened sufficiently after firing and before the gas pressure drops materially so as to impart a substantial force to the compensating piston 22. In this event, in a gun of the type shown herein, the port 23 is so located that it will be fully opened or uncovered before the bolt and short cartridge case 12 have moved one-twentieth of an inch in recoil from their firing position; however, such distance depends on' the size of the firearm, the length ofthe barrel, and other factors and may vary widely in different guns.

Such initial shrouding is usually unnecessary,.however, since the lead projectile from the/regular" .38 Special cartridge is not materially deformed or softened before it clears the gas port. The system illustrated in the drawings does not employ initial shrouding in the short cartridge case 12, before actuation of the compensating piston, but such system is more positive and may be preferable for ordinary ammunition whose projectiles have little tendency to enter the gas port. Ofcourse, initial shrouding is necessary with the .357 Magnum and stronger cartridges to render the compensating device 19 inefiective during firing of such cartridges.

The mouth of each cartridge case'12a' is strong enough to close the port 23 and to resist the flow of the high pressure powder gases into the booster chamber even where the port has a substantial diameter. In the gun shown herein, the gas port 23 has a diameter of about one twentieth of an inch, and test firings indicate that standard .357 Magnum cartridge cases are not deformed atthe gas port when fired in a test gun having that size gas port or'are notdeformed sufiiciently so that any deformation can be measured after ejection of the cartridge case.

While the self-actuating firearm illustrated in the drawing's is intended to handle standard .38 Special and .357

Magnum cartridges, it may also fire other cartridges which'fit'in the barrel, such'as .38 Special and .357 Magnum' cartridges With metal-penetrating loads and fiat-nosed bullets. Such gun may, for example, fire a Remington Highway Patrol cartridge which uses a 110- grain special bullet (standard is 158 grains) or a Western Super-X .38 Special metalpenetrating load which delivers a ISO-grain bullet at 1170 feet per second (instead of a standard l58-grain bullet at 870 'feet per second);

Figure 4 shows in schematic form a simple blowback firearm of the type shown in Figs. 1 to 3 but incorporating a modified form of compensatingdevice 19a'wh'erein the pressure of the powder gases is'applied directlyto the face of the bolt instead of indirectly through a separatepiston. The self-actuating firearm of Fig. 4 includes a receiver 1a, a forwardly extending barrel 2a rigidly connected to the receiver, an externally cylindrical bolt 3amounted for longitudinal reciprocation in the receiver parallel to the axis of the barrel, and a housing 21a for the compensating device 19a, the elements of said me and corresponding to the receiver 1, the barrel 2, the bolt 3,' the driving spring 4, and the housing 21 of'the firearm shown in Figs. 1 to 3. Except for the compensating device 19a, the firearm shown in Fig. 4 may be the same as the firearm of the other figures and may have the same main actuating mechanism so as'to' fire the high power cartridges d automatically or semiautomatically'without' malfunction when the compensat- As herein shown, the

ing device is inefiective. As shown in Fig. 4, a .38 Special cartridge 10 is in its-normal firing position in the counterbore 7 with its base 14 in the circular bolt recess? and its bullet "11 extending beyond the annular shoulder 8 into thebore'6,and the bolt 3a is in its advanced position with its front face 31 contiguous to the rear face 16 of the barrel 2a so as to hold the rim face 15 of the cartridge against the face 16.

The compensating device 19a is provided with an auxiliary pneumatic reciprocating booster motor 20a which functions like the booster motor 20 to prevent malfunctions when the low power cartridges 10 are fired. The motor is supplied with high pressure'gases from the interior of the barrel by an externally cylindrical gas tube 22:: which has a press fit in the housing 21a and by a radial gas port 23a. The gas port 23a may be of the same diameter as the gas port 23 of the compensating device 19 and is located at the same place in the counterbore '7 so as to be open and adjacent the forward edge 18 of the short cartridge case 12 when the cartridge 10 is fully chambered as shown in Fig. 4 and so as to be closed when the more powerful cartridge 10a is fully chambered. An opening 33 is left in the housing 21a after drilling the port 23a, but a plug for this opening is unnecessary since the opening is closed by the tube 22a. If desired, a drive-out hole 34 may be provided in the housing 21a to facilitate removal of the gas tube 22a. The'tube 22a is provided with an axial passage 35 opening to the rear end of the tube and a radial passage 36 which registers with the port 23a to establish communication between said axial passage and the barrel whereby high pressure gases may be admitted to the face of the bolt 3a when the port 23a is opened.

It will be noted that modification-of the operation of the main actuating mechanism of the firearms shown in the drawings is effected in a similar manner in the compensating devices 19 and 19a by employing high pressure gases from the barrel to apply a pressure or force against the face of the barrel, the gases admitted to the booster motor 20 by the gas port 23 applying the force to the bolt indirectly through the piston 22 and the gases admitted to the booster motor 20a by the gas port 23a engaging the face of the bolt to apply the force directly. It will therefore be apparent that a booster effect may be obtained in the compensating device 19 even if the piston 22 is omitted so that the high pressure gases admitted to the booster chamber act directly on the front face 31 of the bolt. However, it is preferable to design the bolt 3a and the gas tube 22a to form an expansible pressure chamber of a predetermined size, whereby the force obtained on the bolt from a given cartridge is predictable, and to vent the receiver In so that a high pressure is not obtained against the front face 31 of the bolt.

As herein shown, the receiver 1a is provided with one or more vent openings 37 to prevent the buildup of high pressure in the receiver between the faces 16 and 31 which would substantially modify the operation of the main actuating mechanism. The bolt 3a is provided with an internally cylindrical cup-shaped recess 38 in axial alinement with the cylindrical tube 22a and of substantially the same diameter, and the tube has a rear end portion 39 which projects into said recess. The recess 38 provides a cylinder for the auxiliary compensating motor 20a and an expansible pressure chamber or booster chamber 26a for receiving high pressure gases from the port 23a and the passages 35 and 36 of the gas tube. The end portion 39 provides a stationary or fixed piston for the motor 20a which slidably fits in the movable cylinder 38 and also acts as a piston valve to open and close communication between the pressure chamber 26a and the vent or vents 37. When the piston-valve formed effectively seals the chamber 26a of the booster motor 50' that a high" pressure may build up "in said chamberagainst the face of the bolt at the bottom of the recess 38 to force the bolt rearwardly. However, the rearward movement of the bolt opens the piston-valve or moves the cylinder 38 to a pressure-releasing position before the bolt travels to its rearmost position so that the chamber 26a is vented to atmosphere through the opening 37.

A construction of the type shown in Fig. 4 is economical to manufacture and provides an excellent firearm due to simplicity and compactness. The compensating device 19a performs the functions of the device 19 of Figs. 1 to 3 and has additional advantages due to the valve action of the piston portion 39 in the recess 38. By properly selecting the length of the piston portion 39 and the depth of the cup-shaped recess 38, the chamber 26a of the compensating motor may be vented to atmosphere after the bolt has moved a predetermined distance in recoil. For example, the piston-valve may begin to open substantially when the long-cartridge case begins to open the gas port 23a so that the compensating motor 20a is ineffective even when the bolt overtravels during the period of high pressure after the firing of a long cartridge such as the cartridge a. The portion 39 of the gas tube and the recess 38 may be designed, for example, substantially as shown in Fig. 4, so as to reach an open or pressure-releasing position relative to the bolt and to initiate the release of pressure in the chamber 261: when the bolt and a long cartridge case 12a carried thereby reach a position wherein the port 23a is partially or fully open. With this construction, regardless of the position of the bolt at or after firing, the compensating device will have no substantial eifect on the operation of the main actuating means of the selfactuating firearm unless the cartridge case is shorter than the case 12a.

It will be apparent that the compensating device of the present invention may have various other forms and may be applied to various types and sizes of self-actuating firearms in addition to those specifically described herein; and it will be understood that, in accordance with the provisions of the patent statutes, variations and modifications of the specific devices shown and described herein may be made without departing from the spirit of the invention.

Having described my invention, I claim:

In combination, a short cartridge, a compensating device, a gas-actuated firearm having a forwardly extending barrel with a firing chamber at its breech end sufficiently long to receive either said short cartridge or a long cartridge, said short cartridge having a case with a radially projecting rim at its base that engages the rear portion of the barrel to limit forward movement of the case, said barrel having a counterbore at the rear of the barrel forming said firing chamber, means comprising a breech bolt mounted for movement toward and away from the breech end of said barrel for closing the breech end of said barrel, said last-named means including actuating means for moving the bolt away from said barrel a distance greater than the length of the cartridge case in response to firing of a cartridge, said compensating device comprising a gas cylinder and a compensating piston in said cylinder spaced from said firing chamber, said cylinder and piston defining an expansible booster chamber spaced from said firing chamber for receiving the high pressure gases from said firing chamber, said piston being operably engaged with said actuating means to cause movement of the same relative to the barrel, said gas cylinder being laterally fixed with respect to said barrel, and passage means for supplying motive fluid from the barrel to said expansible chamber comprising a gas intake port in said counterbore having a diameter not substantially greater than 0.05 inch and occupying a very small fraction of the circumference of said counterbore, said port being spaced forwardly of the rear end of the barrel a greater distance than said short cartridge case and less than the length of said chamber occupied by a long cartridge case.

References Cited in the file of this patent UNITED STATES PATENTS 2,388,396 Eklund Nov. 6, 1945 2,476,232 Williams July 12, 1949 FOREIGN PATENTS 214,505 Great Britain Apr. 24, 1924 453,658 Germany Dec. 15, 1927 300,428 Italy Sept. 8, 1932 

